Nanocoatings of clay and creep of the San Andreas fault at Parkfield, California

Research Article| July 01, 2010 Nanocoatings of clay and creep of the San Andreas fault at Parkfield, California A.M. Schleicher; A.M. Schleicher 1Department of Geological Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109, USA Search for other works by this author on: GSW Google Scholar B.A. van der Pluijm; B.A. van der Pluijm 1Department of Geological Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109, USA Search for other works by this author on: GSW Google Scholar L.N. Warr L.N. Warr 2Ernst-Moritz-Arndt Universität Institut für Geographie und Geologie, F. Ludwig-Jahn-Strasse 17A, D-17487 Greifswald, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information A.M. Schleicher 1Department of Geological Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109, USA B.A. van der Pluijm 1Department of Geological Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, Michigan 48109, USA L.N. Warr 2Ernst-Moritz-Arndt Universität Institut für Geographie und Geologie, F. Ludwig-Jahn-Strasse 17A, D-17487 Greifswald, Germany Publisher: Geological Society of America Received: 02 Feb 2010 Revision Received: 11 Feb 2010 Accepted: 26 Feb 2010 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2010 Geological Society of America Geology (2010) 38 (7): 667–670. https://doi.org/10.1130/G31091.1 Article history Received: 02 Feb 2010 Revision Received: 11 Feb 2010 Accepted: 26 Feb 2010 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation A.M. Schleicher, B.A. van der Pluijm, L.N. Warr; Nanocoatings of clay and creep of the San Andreas fault at Parkfield, California. Geology 2010;; 38 (7): 667–670. doi: https://doi.org/10.1130/G31091.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Mudrock samples were investigated from two fault zones at ∼3066 m and ∼3296 m measured depth (MD) located outside and within the main damage zone of the San Andreas Fault Observatory at Depth (SAFOD) drillhole at Parkfield, California. All studied fault rocks show features typical of those reported across creep zones with variably spaced and interconnected networks of polished displacement surfaces coated by abundant polished films and occasional striations. Electron microscopy and X-ray diffraction study of the surfaces reveal the occurrence of neocrystallized thin film clay coatings containing illite-smectite (I-S) and chlorite-smectite (C-S) minerals. 40Ar/39Ar dating of the illitic mix-layered coatings demonstrated Miocene to Pliocene crystallization and revealed an older fault strand (8 ± 1.3 Ma) at 3066 m MD, and a probably younger fault strand (4 ± 4.9 Ma) at 3296 m MD. Today, the younger strand is the site of active creep behavior, reflecting a possible (re)activation of these clay-weakened zones. We propose that the majority of slow fault creep is controlled by the high density of thin (<100 nm thick) nanocoatings on fracture surfaces, which are sufficiently smectite-rich and interconnected at low angles to accommodate slip with minimal breakage of stronger matrix clasts. Displacements occur by frictional slip along particle surfaces and hydrated smectitic phases, in combination with intracrystalline deformation of the clay lattice, associated with extensive mineral dissolution, mass transfer, and residual precipitation of expandable layers. The localized concentration of smectite in both I-S and C-S minerals contributes to fault weakening, with fracturing and fluid infiltration creating new nucleation sites for neomineralization on displacement surfaces during continued faulting. The role of newly grown, ultrathin, hydrous clay coatings contrasts with previously proposed scenarios of reworked talc and/or serpentine phases as an explanation for weak fault and creep behavior at these depths. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.